1. Technical Field
This invention relates to an inkjet printing press for printing sheets with inkjets.
2. Background Art
Inkjet printing presses are known which have an inkjet head provided at the outer periphery of a rotary printing drum to jet ink against a sheet held on the peripheral surface of the printing drum for printing.
With such an inkjet printing press, the smaller the gap between the sheet and the inkjet head having a plurality of nozzles, the less the interference to ink dots jetted from the nozzles and thus the better the printing quality. But, if the gap is too small, the sheet might contact the inkjet head, thus causing smudges or accidents. Practically, the press is used with the inkjet head in proximity to the sheet to a limit where there is no trouble such as mechanical contact while the inkjet head and the sheet are moving relative to each other for printing. Because this gap is normally extremely small, if the sheet is not held properly on the peripheral surface of the printing drum, accurate printing is impossible and the sheet can contact the inkjet head, thus causing smudges on the print surface of the sheet.
Therefore, with such a press, it is necessary to hold the sheet stably on the printing drum so that the entire surface of the sheet is kept in close contact with the peripheral surface of the printing drum.
As means for holding the sheet, the following two are known. One involves attracting the sheet directly on the peripheral surface of the printing drum by static electricity or air suction, and the other involves holding the ends of the sheet by clamp strips or divided clamps.
With the method of holding a sheet by surface attraction such as by static electricity as disclosed in JP Patent 2868723, the sheet holding force is strong in the transverse direction but weak in the vertical direction. Thus, in printing a firm sheet, attraction at the sheet end tends to be insufficient, and if the sheet is thick, the sheet end tends to separate from the peripheral surface of the printing drum. Also, in order to hold a sheet being fed at high speed, large static electricity is required. This may cause electrical discharge with an inkjet press in which the inkjet head has to be provided in proximity to the printing drum.
Also, with the method of holding the sheet by negative pressure as disclosed in JP Patent 2868723, it is possible to attract the sheet onto the printing drum properly at a closed area such as at the center of the sheet, but at the peripheral portion of the sheet, because air is sucked from sheet ends, negative pressure cannot be maintained, so that sheet ends cannot be held effectively. Further, if the sheet is absorptive such as paper, negative pressure applied to the sheet may cause the ink applied by the inkjet head to permeate into the sheet. This results in change of ink color depending on the degree of permeation and makes it difficult to provide stable printing quality.
On the other hand, with the method of holding the sheet mechanically by clamps, the sheet ends are prevented from separating from the drum surface because the sheet is clamped at its ends. But there are the following problems.
That is, because ordinary clamps are of such a structure that they open and close radially of the printing drum, they cannot impart a tensile force to the sheet. Thus, the sheet tends to be loose and lift off the drum surface, so that it is not possible to obtain a stable mounting state in which the entire sheet is in close contact with the drum surface.
As a device which uses clamps to impart tension to the sheet, the device disclosed in Japanese patent publication 7-195780A is known. But because it imparts tension by dividing a portion of the rotary drum corresponding to the trailing end of the sheet so as to be movable, it is not suited for use with general-purpose presses which print sheets of different sizes at high speed.
Also, because the clamps holds the sheet while pulling both ends of the sheet extending axially toward the center of the drum, if the sheet is thick, the clamps tend to run obliquely on the sheet ends and warp on the sheet with the central portions of the clamps protruding radially outwardly from the drum surface. Thus, the clamps have to be rigid and thick. In this arrangement, it is difficult to assure a gap of proper size between the inkjet head and the peripheral surface of the printing drum.
Also, as disclosed in JP Patent 2559043, a sheet trailing end clamp has been put to practical use which is divided into a plurality of parts axially of the printing drum. But because it is adapted to clamp only part of the trailing end of the sheet, if the sheet supplied has a bent end, the clamp can not hold the sheet. Also, with this type of printing drum, the drum surface is divided circumferentially into rings, and the divided clamps are movably held between the rings, thereby holding sheets of different sizes. But in order to make smooth the drum surface on which divided sheets have to be held in a smooth state, a complicated mechanism requiring high accuracy is needed, and thus the device tends to be very expensive.
An object of the present invention is to provide an inkjet printing press which can hold sheets extremely stably on the peripheral surface of the printing drum and thus can print at high speed with high accuracy.